Until now, metal substrates for industrial use have been subjected to chemical conversion treatment for treating the base surface to improve corrosion resistance and adhesion. However, chemical conversion treatment solutions contain large amounts of various ionic components, as well as large amounts of heavy metal components, such as zinc, nickel, and manganese, to improve the properties of the resulting chemical conversion coating film.
When electrodeposition coating is performed using a cationic electrodeposition coating composition directly after the chemical conversion treatment, it is known that excess chemical conversion treatment solution adhered to or deposited on the metal substrate adversely affects the electrodeposition coatability, finished appearance, corrosion resistance, and the like.
Therefore, a typical coating line includes a water-washing step, which requires a considerable number of steps and time for degreasing treatment, chemical conversion treatment, first water-washing, second water-washing, pure-water washing, electrodeposition coating, first water-washing, second water-washing, pure-water washing, and bake-drying, as shown in FIG. 1. Furthermore, collection, filtration, treatment, disposal, etc., of wastewater from the water-washing step also require massive equipment and costs.
Patent Literature (PTL) 1 discloses a method for forming a multilayer coating film that enables process-streamlining and space-saving. In this method, electrodeposition coating is performed without performing water-washing after chemical conversion treatment. PTL 1 discloses that even when a chemical conversion treatment solution is brought as a contaminant into an electrodeposition coating composition in the subsequent step, it is possible to obtain a coated article with excellent finished appearance and excellent corrosion resistance, without affecting electrodeposition coatability and coating film properties. However, when water-washing is not performed, and if a solution having an electrical conductivity higher than a certain level is adhered to and/or deposited on the substrate, sufficient corrosion resistance and finished appearance might not be achieved.
Patent Literature (PTL) 2 discloses a method for performing chemical conversion treatment by dipping treatment using an acidic zinc phosphate aqueous solution. In relation to this, PTL 2 defines the sodium ion concentration in the zinc nitrite aqueous solution used as an accelerator. PTL 2 discloses in paragraph [0016] that when the sodium ion concentration is 10,000 ppm on a weight basis in a chemical conversion treatment tank, an excellent chemical conversion coating film is obtained. However, when sodium ions are contained at such a high concentration, the electrical conductivity is also high, and the water-washing step cannot be omitted. It is thus difficult to achieve process-streamlining and space-saving.
Patent Literature (PTL 3) discloses a method for chemical conversion treatment that is capable of producing a high-quality chemical conversion film. In relation to this method, PTL 3 discloses in paragraph [0063] that the electrical conductivity of the chemical conversion treatment solution is controlled to about 10 to 200 mS/cm (10,000 to 200,000 μS/cm). When the chemical conversion treatment solution has such a high electrical conductivity, sufficient washing is a necessity, and the washing step cannot be omitted. Therefore, achieving process-streamlining and space-saving is difficult.